Method to integrate regenerative rankine cycle into combined cycle applications

ABSTRACT

A system is disclosed that incorporates a regenerative Rankine cycle integrated with a conventional combined cycle. An added duct firing array, typically located after the combustion turbine exhaust and before the conventionally designed Heat Recovery Steam Generator (HRSG), is used to boost enthalpy of said exhaust. An added heating element downstream of the firing array provides sufficient heating for sensible heating, evaporation and superheating of feedwater that has been previously heated by feedwater heaters as part of a regenerative Rankine cycle. In practice, the condensate stream from the condenser is bifurcated such that a dedicated feedwater flow is directed to feedwater heaters. After further heating in the added heating element, the superheated steam, at the same pressure and temperature as the main steam, is now mixed with the main steam prior to turbine entry. The condensate is directed to the HRSG to be heated in conventional fashion.

U.S. PATENT DOCUMENTS

-   4,829,938 . . . Motai, et at-   4,961,311 . . . James-   4,976,100 . . . Lee-   5,799,481 . . . Fetescu-   6,363,711 . . . Aktiengesellschaft

TECHNICAL PAPERS AND PUBLICATIONS GE Combined Cycle Product Line andPerformance GER-3574G by D. L. Chase and P. T. Kehoe Comparison of PowerEnhancement Options for Greenfield Combined Cycle Power Plants, ThomasC. Tillman, February 2004, Rev. 2 Economic and Technical Considerationsfor Combined Cycle Performance Enhancement Options by Chuck Jones andJohn A. Jacobs III GE Power Systems GER-4200 Introduction to theComplementary Fired Combined Cycle Power Plant, Power-Gen International2006, Siemens CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefits of the U.S. Provisional PatentApplication Nos. 61/741,800 entitled “Method to integrate regenerativerankine cycle into combined cycle applications” filed on Jul. 27, 2012and 61/741,997 filed on Aug. 1, 2012 entitled “Method to integrateregenerative rankine cycle into existing combined cycle applications”and 61/742,633 entitled “Method to integrate solar regenerative rankinecycle into combined cycle applications” filed on Aug. 14, 2012. Theseprovisional applications are incorporated by reference herein in theirentirety.

BACKGROUND OF THE INVENTION

Combined cycle power plants have come of age due to the advances incombustion turbine technology and, most recently, due to the new naturalgas recovery technology of “fracking”. Fracking has significantlyincreased the gas reserves of the United States and has significantlylowered the cost of gas recovery. The state of the combustion turbinetechnology and the availability of long term and relatively low costnatural gas has made the combined cycle the prominent choice for bothfuture generation needs to serve new loads and to replace coalgeneration in the near and mid-term future.

Early applications for combustion turbines were aero derivative modelswhich were, essentially, modified jet engines originally designed foraircraft and modified for land base use. However, the design of thistype of technology, i.e. combustion turbines, gradually became specificto the needs of the electric utility industry such that by the 1970′sspecific combustion turbines with characteristics specifically designedto optimize performance in combined cycle operation were commerciallyavailable.

A combined cycle can be described in two parts; the “top” cycle which isthe combustion turbine utilizing a Brayton cycle, and the “bottom” cyclewhich is the Rankine cycle. Shaft power is initially generated throughthe use of combustion turbines; the turbine section of the combustionturbine used for land base power generation is designed such that thereis no thrust as all developed power is recovered in the shaft; however,there is still significantly high exhaust temperature which, instandalone applications, is wasted. The “bottom” cycle of a combinedcycle is a Rankine cycle which uses the waste heat from the combustionturbine. The turbines used in combine cycle applications have not beendesigned necessarily to be the most efficient in a standaloneconfiguration, but rather to be the most efficient when used in tandemwith a bottoming Rankine cycle. Typically, these types of combustionturbines normally have a low pressure ratio which results in a highexhaust temperature. The high exhaust temperature is beneficial to theRankine cycle which, in tandem use with the combustion turbine, canproduce combined overall efficiencies in the 60% range.

Increasing the efficiency of a combustion turbine typically requireshigher firing temperatures at the turbine inlet and higher pressureratio to use the higher thermodynamic availability resulting from thehigher firing temperature. However, while increasing the firingtemperature without a commensurate increase in the pressure ratio mayminimally increase the efficiency of the turbine, the higher exhausttemperature resulting from the higher firing temperature cansignificantly increase the efficiency of the Rankine cycle. Inaccordance with the second law of thermodynamics, the efficiency of anyheat cycle can be expressed as:

Efficiency=1=(T _(L) /T _(H))

Where T_(L) is the low temperature of the working fluid, i.e. the lowtemperature of the steam in the cycle, where heat is exhausted to theheat sink. T_(H) is the high temperature of the working fluid, in ourcase, steam, and is the point where expansion of the working fluid isused to produce work.

Consequently, it is always thermodynamically preferable to have theworking fluid to be expanded at the highest possible temperature. Inorder to achieve a high steam temperature, typically around 1050 F, ahigh exhaust temperature is required; this exhaust temperature must behigher than the operating steam temperature in order to affect heattransfer. It is also noted that for a high Rankine cycle efficiency, thesteam must be expanded to the lowest possible temperature and pressure.Typically the temperature is around 115 F at about 1.5 psia or so.However, herein lays a problem for an efficient combined cycle.

By expanding and condensing the steam to a low temperature, aregenerative Rankine cycle is not possible for a conventional combinedcycle configuration. In order to achieve a low stack gas temperature,low feedwater temperature must be supplied to the waste heat boiler. Forexample, if the feedwater is heated through regeneration to atemperature of, say, 500 F then it is impossible for the stack gastemperature to be lower than 500 F and, in fact, since a temperaturedifference must be maintained in order to achieve heat transfer (usuallya minimum of 50 F or so), then the stack temperature must exit at around550 F and this hot gas represents an enthalpy loss to the overall cycle.Therefore, feedwater heating, if any, can only be used sparingly inorder to maintain a sufficiently low feedwater temperature in order toensure there is no unreasonable stack loss.

To date, turbine manufacturers have concentrated on increasing firingtemperatures of the combustion turbines for increased efficiencies; buthigh firing temperature requires enormous research and development costsas well as costly material and blade cooling methods. The noveltyproposed herein goes back to the basics and proposes an alternative thatincreases the efficiency of the Rankine cycle not through higheroperating working fluid temperatures but employing regenerative heatingto increase the Rankine cycle efficiency.

Overall, the energy consumption in the United States has declinedslightly over the last 5 years and much of this decline can beattributed to the overall economic decline of the past several years.However, domestic production has still increased by about 3% per yeardue to a decrease in the importation of electricity from Mexico andCanada. Overall, in the next ten years, electric consumption in theUnited States is expected to grow incrementally at about 1 to 1.5% peryear. Even though this is a small number, the total installed capacityin the United States in 2010 was about 1,140 GW's. Therefore, even a 1%increase would require construction of about twenty 500 MW power plantsevery year. And this does not include the replacement capacity due toaging plants, and, in particular, aging coal plants.

There is a significant market driven by the aging coal plants in thiscountry. Over the next 10-15 years, dozens of coal units will bereplaced with gas-fueled combined cycle units. It is unlikely that thepower plant operators will walk away from an existing power plant sitewhich has high value infrastructure including transmission and waterrights as well as a certain ease of permitting since development wouldoccur on an already despoiled plant site. There is significantdifficulty in developing a new coal plant since coal has increased inprice and natural gas has decreased. In addition, the combined cycle isabout 40-45% more efficient than a coal plant and the capital cost isabout ⅓ the cost of a coal plant. And this price differential does notinclude the cost of greenhouse gas (CO₂) clean up which would addconsiderably to the cost of coal generation.

Greenhouse gases will be a significant driver not only for renewableenergy resources but also for combined cycle plants as well. Combinedcycle plants emit less than 50% greenhouse gas than a similar size coalplant operating at the same capacity factor. Green house gas reductionis a significant driver for the construction of combined cycle powerplants. Consequently, a low cost and highly efficient Regenerative cycleintegrated in a combined cycle novelty will be received favorably in thecommercial markets.

SUMMARY OF THE INVENTION

This novelty firstly adds an additional duct firing array into the CTexhaust stream and before the Heat Recovery Steam Generator (HRSG).Then, additional heating elements are placed into the duct immediatelydownstream of the added duct firing array and before the HRSG. With thisadditional ability to add and extract heat, this novelty supplies thenecessary and separate enthalpy to a separate regenerative cycle. Thisis accomplished by creating a separate and designated stream offeedwater mass flow rate by dividing the flow from the condenser hotwell(condensate/feedwater) to allow harvesting of the additional enthalpyresulting from proposed additional duct firing array. This fractionalflow from the condenser is preheated with extraction steam from thesteam turbine and then the heated feedwater captures the additionalenthalpy provided by the additional duct firing array to provide mainand reheat steam. Steam is produced commensurate with the pressure andtemperature of the steam produced by the HRSG such that said steam canbe mixed with the HRSG produced steam.

The remaining fractional flow from the condenser is pressurized tofeedwater and directed to HRSG in conventional manner. Consequently, ina conventional arrangement of feedwater flow to the HRSG, the feedwaterflow must be kept at low temperature prior to entering the HRSG in orderto ensure that the stack gas temperature does not rise. The fractionalflow that is pre-heated by extraction steam is a separate feed and isnot impacted and does not affect the low temperature feedwater flow tothe HRSG.

Thus, full regeneration of the Rankine cycle loop through the use ofturbine steam extraction and feedwater heaters can be achieved with hightemperature feedwater being directed to the additional heating elementsfor main and reheat steam production. Typically, in a standaloneregenerative Rankine cycle, the mass ratio of the total steam extractionflows to the main throttle steam flow is in the order of 0.35 or so(depending on the number of extraction ports and commensurate number offeedwater heaters) to fully utilize regeneration and to pre-heat thefeedwater as much as possible.

While separate steam turbines could be used for the non-regenerative andregenerative steam produced, the common configuration would be toutilize a common turbine and co-mix the two steam flows. Only thatportion of the steam flow generated through the regenerative processwould be part of the regenerative Rankine cycle; the feedwater flowthrough the feedwater heaters would be equal to the amount ofregenerative steam produced. Accordingly, this dedicated flow offeedwater could be raised close to the saturation temperature of theoperating pressure of the HRSG. Limitations would ensue based on amountof total flow of main steam throttle flow to the dedicated feedwaterflow for duct firing. In traditional regenerative cycles, regenerationis normally limited by the amount of heat that can be transferred fromthe main throttle steam flow; in this novelty, the limitation can be theamount of heat absorbed by the feedwater stream. In any case, theheating of the independent feedwater flow for duct firing closer to thepoint of saturation may result in a gain in thermodynamic efficiency.

This invention also benefits from the operational duality of thetraditional duct fired concept using condensate, pressurized tofeedwater, fed directly to the HRSG and the dedicated feedwaterregenerative stream, pressurized to feedwater, fed to the additionalheating elements located downstream in the CT exhaust and upstream ofthe HRSG. By switching from this novelty's concept during plantoperation and running the additional heating surfaces “dry” or withminimal cooling steam flow in the additional heating elements, thetraditional method of duct firing can be implemented with concurrentincrease in condensate flow, which would yield higher capacity althoughat lower efficiency. Accordingly, when efficiency is preferred, theplant can be operated in the enhanced regenerative mode as describedherein; when higher capacity is required the switch can be made totraditional duct burning.

Another variation or additional embodiment of this novelty is to addreheat with the regeneration feature. In this manner, a dedicated ductburner array and heating element are used for reheating. This techniqueallows for reheating back to the original main steam temperature withoutimpacting the stack gas temperature or design parameters in the HRSG.

It is noted that this novelty can be applied to new installation or toexisting regenerative Rankine cycle installations. In particular, coalplants that are near end of life operation could be repowered utilizingthe existing steam turbine generator, feedwater train and associatedpiping, and equipment as well as the indigenous infrastructure such assite and transmission. In this embodiment, at least one combustionturbine with at least one HRSG could be used to incorporate the existingcoal plant's equipment.

BRIEF DESCRIPTION OF THE DRAWINGS

Drawing 1 is a sketch that diagrammatically shows the proposed concept.The drawing shows an inner feedwater loop, shown in dotted lines,employing feedwater heaters supplying additional feedwater flow in adesignated flow path such that additional heating elements and addedduct firing results in a separate regenerative Rankine cycle. The ductfiring shown is an added array and is not to be confused withconventional duct firing used to increase the steaming capacity of theHRSG. The novelty's added duct firing array does not impact or impedethe operation of the conventional duct firing and can be used in tandemwith the proposed novelty. The novelty's proposed additional duct firingdoes not increase the feedwater flow rate from the condenser directly tothe HRSG to produce more steam; this novelty proposes a separate loopmethod allows the feedwater to be preheated in a separate loop usingextraction flows from the steam turbine with additional enthalpy addedfor steam production using a dedicated duct burner array.

Drawing 2 is similar to Drawing 1 but shows the additional embodiment ofreheating that would be available, if deployed, under this novelty. Inthis scheme, the cold reheat steam is bifurcated with the majority ofsteam flowing to the separate and designated heating elements and theremaining steam flow used for regenerative heating in the first pointheater.

Drawing 3 is similar to Drawing 1 but shows the additional embodiment ofadding solar thermal energy to the fuel mix which is used to generatesteam in the regenerative portion of the overall cycle.

Drawing 4 is a sketch that diagrammatically shows the application of theregenerative ranking cycle integrated with a combined cycle having anon-extraction steam turbine.

DETAILED DESCRIPTION OF THE INVENTION

The numbers and data shown are general approximations only in order tomore fully delineate the principles of the proposed novelty and theoverall flow schematic should not be construed as a final thermodynamicanalysis. Referring to Drawing 1, if we assume a closed operatingRankine cycle, condenser 1 condenses the steam flow 18 from the lowpressure steam turbine 12. This novelty separates that amount ofcondensate into two streams 2 and 3 where stream 2 is the additionalmass flow rate used for regeneration and absorbs the heat from steamextractions from appropriate ports in the extraction turbine. Inpractice, the fraction dedicated to the regenerative portion of thecondensate flow 2 from the condenser is, typically, about 40-45% oftotal condensate flow. However, these values can be adjusted for cycleoptimization. The pre-heated feedwater 7 is shown in Drawing 1 as adedicated feed to the separately fired heating element 8. The amount ofcondensate 3 used for non-regenerative cycle operation is fed directlyto the HRSG 9 for feedwater heating, evaporating and superheating andthen directed to the High Pressure (HP) steam turbine 11. Condensate 2flows through the regenerative heater #3 4, then through heater #2 5 andthen completes its pre-heating through heater #1 6. Typically, intraditional Rankine regenerative reheat cycles that are non-critical,the first point heater (heater #1) 6 receives steam extraction from thecold reheat line; this embodiment of reheat is described further underother embodiments. The herein embodiment description assumes that thefirst point heater 6 receives its extraction flow from the cold reheatline from the HP turbine 11. For simplicity, boiler feed pumps and otherassociated flow lines, such as feedwater drip lines, have not beenshown.

The amount of reheating, and the number of feedwater heaters, is aneconomic evaluation whereby the cost of preheating is evaluated againstthe gain in efficiency; typically large coal plants use 7 or 8 heaters;if a new facility is used, an economic evaluation will determine thenumber of feedwater heaters used. Drawing 1 shows only three forsimplicity. While this novelty permits heating close to the saturationpoint, it is assumed here for illustrative purposes that the pre-heatedfeedwater 7 is heated to approximately 500 F. Heating elements 8 providesensible heating, evaporation and superheating required for productionof main steam. All three elements 8 are shown as a single heatingelement for simplicity and would be located upstream of the HRSG 9 anddownstream from the added duct firing 14.

While the exhaust of the combustion turbine 13 is shown as 1160 F, theadditional duct firing 14 adds heat such that the overall gastemperature is now 1540 F. The amount of heat required to eventuallygenerate steam from the feedwater 7 would then bring down the combustionturbine's exhaust gas temperature to approximately the temperature asbefore (1140 F). This is the same temperature that the exhaust gas wouldneed to produce the enthalpy to run the combined cycle without ductfiring 14. In other words, the total flow enthalpy to HRSG 9 isessentially reset to its original enthalpy value and the many heatingelements in the HRSG 9 (which are not shown in Drawing 1 or Drawing 2)are not impacted and the stack temperature remains unchanged.

Referring again to Drawing 1, the feedwater 7 is heated in theadditional heating element 8, the feedwater stream which is nowsuperheated steam 10 is directed to the inlet of the HP steam turbine 11where it is mixed with the main steam produced by the CT exhaust flow inthe HRSG 9 at the same pressure and enthalpy for expansion in the HPturbine 11. It is noted that this example depicts a three pressurecombined cycle and that the low pressure steam 15, and the intermediatepressure steam 16 are directed to the IP/LP steam turbine 12 asappropriate. The main steam 17, the intermediate pressure steam 16 andthe low pressure steam 15 have all been generated with essentially nochanges to the HRSG 9. The primary design parameter proposed in thisnovelty is that the heating of the separated and designated regenerativefeedwater 7 is performed solely with the added heating elements 8 andthe added duct firing 14 in the duct upstream of the conventionallydesigned HRSG 9 where the said HRSG design is, essentially, unalteredand the stack temperature 23 remains, essentially, unchanged.

Referring to Drawing 2, the addition of a reheat section is shown inconjunction with the production of main steam produced by the previouslydescribed regenerative Rankine cycle in Drawing 1. The cold reheatworking fluid 24 is a separate loop used to reheat that portion of themain steam that has been generated through a regenerative Rankine cycle.It is noted that the main steam produced by the HRSG using solely thewaste heat of the CT 13 is reheated through the HRSG operation only.Drawing 2 is the same as Drawing 1 except for the addition of thespecific equipment and lines required for reheating of the main steam.In Drawing 2, we follow the assumption that most non-critical Rankinecycles take the first point heater steam extraction 22 from the coldreheat line 19. The remaining fraction of the cold reheat 24 is thendirected to an additional heat element 21 to be reheated locatedupstream of the HRSG 9 such that the reheat steam 20 is reheated to thetemperature of the Intermediate pressure steam (IP) produced by the HRSG9. The reheated steam 20 is directed to the intermediate steam line 16and mixed with the combined cycle's production of intermediate steam anddirected to the IP/LP steam turbine 12. The reheating process does notimpact the stack temperature 23.

The novelty described can, with minor variation, also be used toincorporate solar thermal energy into the regenerative Rankine cycle inconjunction with a combined cycle. Referencing Drawing 3, the method toincorporate solar thermal is described. In this embodiment, solar heatenergy is used to further heat the feedwater 7 after heating iscompleted through feedwater heating using extraction steam. FromCondenser 1 the feedwater stream 2 is preheated in the feedwater heatersusing extraction steam; the heated feedwater 7 is directed to the solarboiler 28 where, depending on the amount of solar energy supplied by thesolar field 29, sensible or sensible and latent heat is added. Undermaximum solar heating supplied, dry steam at the saturation pressure canbe directed to the additional heating element 8. In heating element 8,further heat is added such that superheated main steam 10 at throttlepressure is produced for eventual mix with the main steam 17 produced bythe HRSG 9. This method reduces the amount of heat that duct firing 14is required to add.

This novelty can also be employed to retrofit existing combined cyclepower plants. However, the retrofit design is less efficient than thepreferred embodiment since the amount of extraction heating is reduced;in addition, said heating is added at a greater temperature differencethat increases the entropy losses in the cycle process. But, by usingthe existing infrastructure at an existing plant, cost can be reducedand extra capacity may be added economically. Referring to Drawing 4,the designated feedwater 2 to be pre-heated by extraction steam isdirected to Heater 1 6 where steam from the cold reheat 26 is used asthe said extraction steam 22. This steam line 26 is already in existenceand tapped to provide a fractional steam 22 flow to the feedwater heater6. The heated feedwater 7 is then directed to the separately firedelement 8 where further enthalpy is added and the rest of the cycle isprocessed as described in the preferred embodiment.

1. A method for generating electric power that incorporates the use of aregenerative Rankine cycle with a combined cycle, the method comprisingthe steps of: Bifurcating the condensate from a condenser into two ormore separate condensate feed streams whereby the condensate in at leastone condensate feed stream is pressurized to feedwater and sent directlyto a heat recovery steam generator and the condensate in at least onecondensate feed stream is pressurized to feedwater and sent to at leastone separately fired heating element first being preheated by a one ormore feedwater heaters utilizing extraction steam from an extractionturbine; generating steam in at least one separately fired heatingelement and transferring the steam to an extraction steam turbine havingone or more extraction ports; converting the steam into electricitythrough the use of an extraction steam turbine and generator andextracting some of the steam for heating feedwater.
 2. The method ofclaim 1, wherein additional heat enthalpy is supplied to the separatelyfired heating element and used to boost the temperature and enthalpy ofthe combustion turbine exhaust flow such that there is additionalenthalpy in said combustion turbine exhaust flow to generate steam foruse in a regenerative Rankine cycle.
 3. The method of claim 1, whereinthe separately fired heating element is placed downstream of thecombustion turbine inside the combustion turbine exhaust ducting.
 4. Themethod of claim 1, wherein the separately fired heating element may beconfigured in a “once through” or drum design.
 5. The method of claim 1,wherein the method may be utilized in conjunction with a single pressureor multiple pressure heat recovery steam generator.
 6. The method ofclaim 2, wherein some or all of the additional heat enthalpy supplied tothe separately fired heating element is generated from combusting fuelin at least one duct burner.
 7. The method of claim 2, wherein theadditional heat enthalpy supplied to the separately fired heatingelement may be generated from fossil fuel or non-fossil fuel or acombination of both.
 8. The method of claim 2, wherein substantially allof the additional heat enthalpy supplied to the separately fired heatingelement is utilized to generate steam.
 9. The method of claim 2, whereinsome or all of the additional heat enthalpy supplied to the separatelyfired heating element is supplied through the use of one or more ductburners placed in the combustion turbine exhaust ducting and before theseparately, fired heating element.
 10. A method to generate reheatedsteam utilizing a separately fired heating element in a regenerativeRankine cycle used in conjunction with a combined cycle, the methodcomprising of: Partially expanded steam from the high pressure turbineexhaust is sent to an independent fired heating element to boost saidsteam to a temperature that is compatible with the hot reheat steamproduced by the heat recovery steam generator for mixing with total mixdirected to the intermediate pressure turbine inlet; a duct burner toprovide for the necessary enthalpy into the separately fired heatingelement to reheat the steam is placed downstream of the combustionturbine inside the combustion turbine exhaust ducting.
 11. A method forgenerating electric power that incorporates the use of a regenerativeRankine cycle with a combined cycle, the method comprising the steps of:Bifurcating the condensate from a condenser into two or more separatecondensate feed streams whereby the condensate in at least onecondensate feed stream is pressurized to feedwater and sent directly toa heat recovery steam generator and the condensate in at least onecondensate feed stream is pressurized to feedwater and sent to at leastone separately fired heating element first being preheated by a one ormore feedwater heaters utilizing cold reheat steam from a non-extractionturbine; generating steam in at least one separately fired heatingelement and transferring the steam to a non-extraction steam turbine;converting the steam into electricity through the use of anon-extraction steam turbine and generator.